Microstructures and single mask, single-crystal process for fabrication thereof

ABSTRACT

A single mask, low temperature reactive ion etching process for fabricating high aspect ratio, released single crystal microelectromechanical structures independently of crystal orientation.

This invention was made with Government support under Grant No. DABT63-92-C-0019, awarded by DARPA and Grant Nos. ECS 8805866 and ECS8815775 awarded by the National Science Foundation. The Government hascertain rights in the invention.

This application is a continuation of copending application Ser. No.08/804,826, filed on Feb. 24, 1997, now U.S. Pat. No. 5,846,849 which isa continuation of application Ser. No. 08/312,797, filed on Sep. 27,1994, now U.S. Pat. No. 5,719,073, which is a continuation ofapplication Ser. No. 08/013,319 filed on Feb. 4, 1993, now abandoned,the disclosures of which are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates, in general, to microstructures and to a singlemask process for fabricating them and more particularly, tomicroelectromechanical and microoptomechanical structures-and to asingle-mask process for fabricating complete structures includingreleased, movable elements and connectors such as pads, runners,electrodes, and the like on a substrate.

Recent developments in micromechanics have successfully led to thefabrication of microactuators utilizing processes which have involvedeither bulk or surface micromachining. The most popular surfacemicromachining process has used polysilicon as the structural layer inwhich the mechanical structures are formed. In a typical polysiliconprocess, a sacrificial layer is deposited on a silicon substrate priorto the deposition of the polysilicon layer. The mechanical structuresare defined in the polysilicon, and then the sacrificial layer is etchedpartially or completely down to the silicon substrate to free thestructures.

The initial research into surface micromachining established theviability of the technology. Moving rotors, gears, accelerometers, andother structures have been fashioned through the use of such a processto permit relative motion between the structures and the substrate. Thisprocess relies on chemical vapor deposition (CVD) to form thealternating layers of oxide and polysilicon and provides significantfreedom in device design; however, CVD silicon is usually limited tolayers no thicker than 1-2 μm, since residual stress in thicker layersoverwhelms the structure and causes curling. Thus, although a largevariety of layers can be combined to form very complicated structures,each layer is limited in thickness. In addition, the wet chemistryneeded to remove the interleaved oxide layers often takes tens of hoursof etching to remove, and once released the structures often reattach orstick to the substrate because of static electricity, and this requireselaborate process steps to overcome. The structures made of polysiliconinherently have a crystalline structure which has low breaking strengthbecause of grain sizes, as well as electronic properties which areinferior to single crystal silicon. Furthermore although this technologyis well established, it is not easily scaled for the formation ofsubmicron, high aspect ratio mechanical structures.

In bulk micromachining, a silicon substrate is etched and sculpted toleave a structure. This has typically been done using wet chemicaletchants such as EDP, KOH, and hydrozine to undercut single crystalsilicon structures from a silicon wafer. However, such processes aredependent on the crystal orientation within the silicon substrate, sincethe chemistry etches as much as ten times faster in somecrystallographic planes of silicon than in other planes. Although theshapes can be controlled to some degree by the use of photolithographyand by heavy implantation of boron, which acts as an etch stop, it isdifficult to control the process and accordingly, the type, shape andsize of the structures that can be fabricated with the wet chemical etchtechniques are severely limited. In particular, wet etch processes arenot applicable to small (micron size) structure definition, because theyare not controllable on that scale.

A dry bulk micromachining process which utilizes thermolateral oxidationto completely isolate 0.5 μm wide islands of single crystal silicon isdescribed, for example, in the article entitled "Formation of SubmicronSilicon-On-Insulator Structures by Lateral Oxidation ofSubstrate-Silicon Islands", Journal of Vacuum Science Technology, B6(1), January/February 1988, pp. 341-344, by S. C. Arney et al. Thiswork led to the development of a reactive ion etching (RIE) process forthe fabrication of submicron, single crystal silicon, movable mechanicalstructures wherein the oxidation-isolation step described in the Arneyet al publication was replaced with an SF₆ isotropic release etch. Thisprocess, which allowed the release of wider structures, in the range of1.0 μm, and deeper structures, in the range of 2-4 μm, is described inU.S. patent application Ser. No. 07/821,944, filed Jan. 16, 1992,assigned to the assignee of the present application. As there described,this dry etch process utilizes multiple masks to define structuralelements and metal contacts and permitted definition of small, complexstructures in single crystal silicon, and was easy to implement.However, the second lithography step was difficult to apply to deeperstructures, particularly because of problems in aligning the secondmask. Furthermore, that process relied upon the formation of a silicondioxide layer on a single crystal silicon substrate, but since othermaterials such as GaAs or SiGe do not generate an oxide layer the waysilicon does, the process could not be transferred to such othersubstrate materials.

In copending U.S. patent application Ser. No. 07/829,348, a process forreleasing micromechanical structures in single crystal materials otherthan silicon is described. This process uses chemically assisted ionbeam etching (CAIBE) and/or reactive ion beam etching (RIBE) to makevertical structures on a substrate, and uses reactive ion etching (RIE)to laterally undercut and release the structure. The process utilizesmultiple masks, however, and thus encountered similar problems to thesilicon process described above in the formation of deeper structures,and in the alignment of the second mask.

The use of single-crystal materials for mechanical structures can bebeneficial, since these materials have fewer defects, no grainboundaries and therefore scale to submicron dimensions while retainingtheir structural and mechanical properties. In addition, the use ofsingle-crystal materials, particularly single crystal silicon andgallium arsenide, to produce mechanical sensors and actuators canfacilitate and optimize electronic and photonic system integration. Forexample, single crystal silicon structures having a very small mass canresonate without failure at 5 MHz for 2 billion cycles with avibrational amplitude of plus or minus 200 nm. Accordingly, thefabrication of submicron mechanical structures with high aspect ratioswould be highly desirable.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide animproved self-aligned dry-etch process for the fabrication ofmicroelectromechanical structures (MEMS).

It is another object of the invention to provide a process forfabricating structures having micrometer-scale dimensions through asimplified, single-mask, dry-etch process.

Another object of the invention is to provide a low-temperature,single-mask, dry-etch process for fabricating high aspect ratio,released structures with micrometer-scale dimensions.

Still another object of the invention is to provide an improved,low-temperature process for the fabrication of microstructures inpreexisting integrated circuit wafers, such as wafers which incorporateelectronic circuitry, and to provide a process for connecting suchstructures to preexisting circuitry.

Another object of the invention is to provide a single-mask,low-temperature, dry etch process for fabricating microstructures onsingle crystal substrates in which circuit elements have beenconstructed by a different process.

A still further object of the invention is to provide a single mask, lowtemperature process for making movable microstructures having deeptrenches with substantially parallel walls and high aspect ratiosindependent of grain orientation in the substrate.

A still further object of the invention is to provide a single mask, lowtemperature process for fabricating mechanical actuator microstructuresmovable within a substrate or movable out of the plane of the substrate.

A still further object of the invention is the provision of movablemicrostructures in single crystal silicon, such microstructures beingfabricated by a dry etch, single mask process, the structures beingusable as sensors, resonators and actuators, inductors, capacitors, andthe like.

A still further object of the invention is the provision of movablemicrostructures on single crystal silicon substrates, the structuresoptionally including membranes, and being used as sensor/actuators,resonators, optical reflectors, and the like movable in the plane of thesubstrate and rotatable out of that plane, such microstructures beingfabricated by a dry etch, single mask process and by selectivedeposition of metal.

Briefly, the process of the present invention is a single-mask, lowtemperature (less than about 300°), self-aligned process for thefabrication of microelectromechanical (MEM) structures, the processallowing the fabrication of discrete MEM devices as well as theintegration of such structures on completed integrated circuit wafers.The process may be used to produce a variety of sensor devices such asaccelerometers, as well as a variety of actuator devices, resonators,movable optical reflectors, and the like either as separate, discretedevices on a substrate, or as components on previously-fabricatedintegrated circuits. The process may be referred to as the SCREAM-Iprocess (Single Crystal Reactive Etch and Metal).

The present SCREAM-I process is a dry bulk micromachining process whichuses reactive ion etching to both define and release structures ofarbitrary shape, and to provide defined metal surfaces on the releasedstructure as well as on stationary interconnects, pads and the like. Theearlier process defined in the above-mentioned copending applicationSer. No. 07/821,944 also permitted fabrication of microstructures, butrequired two masks to define the structural elements and the metalcontacts.

The present invention was developed from that earlier process, butimproves on it by extending the structural depth to about 10 to 20 μm,permitting formation of beam elements 0.5 μm to 5 μm in width,eliminating the second mask/lithography step of the prior process, andallowing all structural elements, including movable elements such asbeams and stationary elements such as interconnects, beams and contactpads to be defined with a single mask so that the metal contacts appliedto the structure are self-aligned.

The process relies, in general, on the formation of a dielectric mask ona single-crystal substrate such as silicon, gallium arsenide, silicongermanium, indium phosphide, compound and complex structures such asaluminum-gallium arsenide-gallium arsenide and other quantum well ormulti layer super lattice semiconductor structures in which movablereleased structural elements electrically isolated from surroundingsubstrate materials and metallized for selective electrical connectionscan be fabricated using a single mask. The structure so fabricated canbe discrete; i.e., fabricated in its own wafer, in which case any of theaforementioned substrate materials can be used, or it can be integratedin a silicon integrated circuit wafer, in which case the substratematerial will be silicon, allowing low temperature processing inaccordance with the invention.

Complex shapes can be fabricated in accordance with the invention,including triangular and rectangular structures, as well as curvedstructures such as circles, ellipses and parabolas for use in thefabrication of fixed and variable inductors, transformers, capacitors,switches, and the like. Released structures are fabricated for motionalong X and Y axes in the plane of the substrate, along a Z axisperpendicular to the plane of the substrate, and for torsional motionout of the plane of the substrate.

In essence, the invention permits fabrication of released structures andthe subsequent metallization of such structures with a single dielectricmask by using that mask to define deep isolating trenches completelyaround the structures, undercutting these structures to selectivelyrelease them and to produce cavities at the bases of surrounding mesas,and then metallizing the exposed surfaces. The undercutting and cavityformation breaks the continuity of the deposited metal, therebyelectrically isolating the metal on released structures and definedmesas from the metal on the bottom of the trenches, and a dielectriclayer isolates the metal from the underlying substrate. The elementsdefined by the trenches are interconnected by the metal layer so thatreleased structures can be electrically connected through the metallayer to pads in the surrounding mesas, with interconnects provided inselected locations and with the interconnects and pads also beingdefined by the trenches.

When the single mask process is carried out on a discrete substrate orwafer, the process can be carried out by either high or low temperatureprocesses, but when the structures are to be integral with existingcircuitry on a wafer, a low temperature process is preferred to preventdamage to the existing circuitry. The process of the invention, informing deep, narrow trenches to define the isolated and releasedstructures produces high aspect ratio structures which can be metallizedon their side walls for high capacitance between adjacent walls. Inaddition, the process permits a deep etching below the releasedstructures to reduce parasitic capacitance between the releasedstructures and metal on the trench floor. The etching process alsoproduces extended cavities in the side walls of the mesas surroundingthe released structures so reduce leakage current between metal on theside walls and on the trench floor. The metallization on the releasedstructures cooperates with metallization on mesa side walls andmetallization on the trench floors to capacitively control and/or sensehorizontal and vertical motion of the released structure.

If desired, it is also possible to carry out additional steps aftercompletion of the single mask processing described above to modify theresulting structure. For example, an additional masking step can permitreduction of the spring constant of a released beam. In addition, amembrane can be added to released structures to increase their weightfor use in accelerometers, and these membranes can be polished for useas movable mirrors. Additional steps may be used to connect metal layersto external circuitry, as by way of vias, and plural layers can befabricated to form superimposed structures.

A wide range of devices can be fabricated utilizing the process of thepresent invention. As noted above, the process is independent of crystalstructure in the substrate, so that essentially any shape can befabricated and released. Thus, single or multiple fingers cantileveredto a side wall of a substrate and extending outwardly over a trenchbottom wall, various grids and arrays can be fabricated and variouselectrical components can be formed. These various structures may bereferred to herein as "beams" or "released beams", it being understoodthat such beams or released beams can be of any desired shape and can besingle or multiple structures.

The metallization of selected walls and surfaces of the beams andsurrounding substrate allows capacitive control and sensing ofhorizontal motion in the released structures while metallization of thetrench bottom wall allows control and sensing of vertical motion byproviding a relatively wide released structure, for example in the formof a grid or plate, supported axially by single beams to a surroundingmesa wall, and by selectively applying a potential between one side orthe other of the plate and the metal on the trench bottom permitstorsional rotation of the plate about the axis of its supporting arms.Such a torsional rotation of a plate has numerous applications; forexample, in optics.

The released structures of the present invention can be in the form of,for example, a single beam which serves as an accelerometer or a sensorand which is movable horizontally and vertically or in the form ofplural beams in side by side parallel arrangement, or in various otherarrays. Plural beam structures can work together, for example movinghorizontally toward and away from each other to form "tweezers" or canhave varying characteristics, such as thickness, to provide varyingresponses and thus to provide a wide range of motion or sensitivity in,for example, an accelerometer. Various grid-like arrays may be providedto add mass or to provide torsional motion as described above, theprocess of the present invention permitting an exceptionally widevariety of arrays.

An example of the use of structures fabricated in accordance with theprocess of the present invention is in torsional resonators fabricatedfrom single crystal silicon. Micro-machined resonators have beenemployed in a wide range of sensor applications, and in suchapplications the most important resonator characteristic is mechanicaldissipation. Often, a sensor application requires operation of aresonator in a gas or liquid medium such as air and because of theirsmall mass, such resonators are expected to have their dissipationdetermined by the surrounding medium. Torsional resonators fabricated ofsingle crystal silicon in accordance with the invention have a lowermechanical dissipation, and thus are valuable in this application.Furthermore, torsional resonators constructed in accordance with thepresent invention undergo motions out of the single crystal siliconwafer plane and are capable of large motions in this direction. Such outof plane motion presents potentially useful actuator and rotating mirrorapplications.

The basic process of the present invention can be outlined as follows.

First, a dielectric layer of oxide or nitride is deposited on a wafer orsubstrate, this layer serving as a mask throughout the remainder of thesteps. A standard PECVD process is used because of its high depositionrate and low deposition temperature. Thereafter, resist is spun, exposedand developed on the mask layer. Standard photolithographic resisttechniques are used to define the desired beams, pads, interconnects andlike structures. Thereafter, the pattern produced in the resist istransferred from the resist to the mask dielectric using, for example,CHF₃ magnetron ion etching (MIE) or RIE.

An O₂ plasma etch may be used to strip the resist layer, and thepatterned dielectric mask is then used to transfer the pattern into theunderlying wafer to form trenches around the desired structures. A deepvertical reactive ion etch (RIE) or chemically assisted ion beam etch(CAIBE) is required for this purpose. Depending on the choice ofstructure height, the trenches may be from 4 to 20 μm deep, withsubstantially vertical, smooth walls.

After completion of the trenches, a protective conformal layer of PECVDoxide or nitride is applied to cover the silicon beams/structures to athickness of about 0.3 μm thick. The conformal dielectric layer coversthe top surfaces of the surrounding substrate (or mesa), the definedstructures, and the sides and the bottom wall of the trench, so it isnecessary to remove the dielectric from the trench bottom wall, as by ananisotropic CF₄ /O₂ RIE at 10 mT. This etch does not require a mask, butremoves 0.3 μm of dielectric from the beam and mesa top surfaces andfrom the trench bottom, leaving the side wall coating undisturbed. As aresult, the beam is left with a top surface layer and a side wall layerof dielectric, with the bottom of the trench being film-free.

A deep RIE or CAIBE is then used to etch the trench floor down below thelower edge of the sidewall dielectric. This etch preferably exposes 3 to5 μm of substrate underneath the dielectric on each side of the beamsand under the dielectric on the walls of the surrounding mesa, and it isthis exposed substrate under the beams and on the mesa walls which is tobe removed during the release step. The release is carried out by anisotropic RIE which etches the substrate out from under the beams orother structures, thus releasing them, and etching the substrate on themesa walls to form cavities. The etch has high selectivity to thedielectric, allowing several microns of substrate to be etched withoutappreciably affecting the protective dielectric coating. After release,the beams are held cantilevered over the bottom wall of the deep silicontrench by their connections to the surrounding mesa at their ends, forexample.

Each released, cantilevered structure has a core of semiconductormaterial such as single crystal silicon and a conformal coating ofdielectric surrounding it on the top surface and side walls. Thestructural beams may be cantilevered at the ends and free-floating inthe center, for example. To activate the structure, either by measuringits motion or by driving it into motion, a metal layer is required.Accordingly, as a final step, an aluminum layer is sputter depositedonto the beam top surface and side walls of the beam, onto the floor ofthe trenches, and onto the top surface and side walls of the surroundingmesa. The structure is now complete and simply needs to be connected tosuitable circuitry to activate it. The circuitry may be on a separatesubstrate or may be formed in the wafer adjacent the location of thebeam prior to fabrication of the beam. It may also be desirable,depending on the application, to add a thin passivation oxide layer 100to 200 nm thick to prevent shorting between moving structures.

In designing and fabricating a single-mask MEM device using the presentinvention, only a single layer of metal is used because of thesimplified nature of the process. Furthermore, in addition tosurrounding the beams, a trench must surround every electricallycomplete set of interconnects, contact pads and capacitive plates. Thistrench serves a dual purpose. It isolates and defines the individualstructured elements such as the beams, pads and conductors and, becauseof the self aligned nature of the process, the trench automaticallypatterns the metal. Released MEM beams normally are narrow to facilitatethe release step and to provide flexibility and light weight, whileinterconnects formed by the same process normally are wider to preventrelease and to provide stable support for the released beam. Beamsdestined for release typically would be about 1 μm wide and preferablyless than a maximum of between 3 and 5 μm. Interconnects typically arenot meant to move, and thus can be wider than 5 μm.

Because of the low temperature requirements of some embodiments, thesimple design rules and the short process times of the presentinvention, the process described herein can be added to conventionalvery large scale integrated circuits without damaging such circuits orinterfering with their construction. One of the benefits of the presentprocess is the ability to define all the electrical components with asingle mask. In prior processes, after every metal deposition, aphotolithography step became necessary to etch back the metal and definethe interconnects. In the present invention, the etch back is notnecessary, for the metal runners are self aligned and self-defined.

BRIEF DESCRIPTION OF THE DRAWINGS

Although the foregoing and additional objects, features and advantagesof the present invention will become apparent to those of skill in theart from the following more detailed description of a preferredembodiment, taken in conjunction with the accompanying drawings, inwhich:

FIGS. 1A-1J illustrate steps 1 through 10, respectively, of the processof the present invention;

FIG. 1K illustrates a modification of Step 9 of the process;

FIG. 1L illustrates a further modification of the process of steps 1-10;

FIG. 2 is a top plan view of a beam and associated contact pads andinterconnects fabricated by the process of FIGS. 1A-1J;

FIG. 3 is a cross-sectional view of the device of FIG. 2, taken alonglines 3--3 thereof;

FIG. 4 is a cross-sectional view of the device of FIG. 2, taken alonglines 4--4 thereof;

FIGS. 5A-5C illustrate optional preliminary steps for a secondembodiment of the invention, wherein a MEM structure fabricated inaccordance with the process of FIGS. 1A-1J is incorporated in anintegrated circuit wafer;

FIGS. 5D-5I illustrated additional steps for the second embodiment ofthe invention;

FIG. 6 is a diagrammatic view of a third embodiment of the invention;

FIG. 7 is a diagrammatic view of a fourth embodiment of the invention;

FIGS. 8A-8L illustrate the steps of a fifth embodiment of the invention;

FIG. 9 is a diagrammatic perspective view of a structure for providingtorsional motion;

FIGS. 10A-10C illustrate steps for producing electrodes for use in thestructure of FIG. 9;

FIGS. 11A-11C illustrate steps for producing a modified form ofelectrodes for use in the structure of FIG. 9;

FIG. 12 is a partial cross-sectional view of FIG. 9 illustrating amodification of the torsional structure; and

FIG. 13 illustrates a structure fabricated in accordance with theprocess of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although a number of variations on the basic process of the presentinvention are described hereinbelow, the basic process is capable offabricating a functioning MEM structure with a single mask. The basicprocess and its variations all rely on the characteristics ofsingle-crystal structures for the substrates, and further depend on theuse of standard fabrication tools, on the use of optical or E-beamlithography and on self-aligned construction. The invention permitsdefinition of high-aspect-ratio structures having walls closely spacedto corresponding walls on a substrate for high capacitance between theadjacent walls. The basic process is illustrated in steps 1 through 10of FIGS. 1A through 1J, respectively, of the drawings, which outline asingle-mask, low-temperature, self-aligned process for makingmicroelectromechanical structures. These figures illustrate thefabrication of a released, cantilevered beam which is free to move leftor right in a generally horizontal path (as viewed in the Figures), andillustrate an adjacent vertical static plate which forms a parallelcapacitor plate to measure the motion of the beam or, in thealternative, to cause motion of the beam.

The illustrated process is implemented with a single mask, with thestructure being fabricated as a discrete element on a wafer or die. Aspreviously noted, a discrete element is defined as an element on a waferwithout electronic circuitry but which has bonding/contact pads thatwill be wire-bonded to external circuitry. Another embodiment of theinvention, discussed below, relates to fabrication of an integratedelement which, as used herein, refers to a structure fabricated on anintegrated circuit chip and directly connected to on-chip circuitrythrough interconnects or vias.

The following Table I outlines the process of the present invention, aswill be described more fully hereinbelow.

                  TABLE I                                                         ______________________________________                                                     Description    Description                                       Step Number  (Single Crystal Silicon)                                                                     (Gallium Arsenide)                                ______________________________________                                        1.  Mask dielectric                                                                            PECVD (1-2 μm)                                                                            PECVD                                                          oxide or nitride                                                                             oxide or nitride                              2.  Photolithography                                                                           Vapor prime    Vapor prime                                                    Spin & bake resist                                                                           Spin & bake resist                                             Expose         Expose                                                         Develop        Develop                                                        Descum         Descum                                        3.  Pattern transfer                                                                           CHF.sub.3 MIE  CHF.sub.3 MIE                                 4.  Resist removal                                                                             O.sub.2 plasma O.sub.2 plasma                                                                (Optional)                                    5.  Trench etch #1                                                                             Cl.sub.2 RIE   Cl.sub.2 CAIBE                                6.  Sidewall dielectric                                                                        PECVD (0.3 μm)                                                                            PECVD oxide                                                    oxide or nitride                                                                             or nitride                                    7.  Clear floor  CF.sub.4 RIE   CF.sub.4 RIE oxide                                dielectric                  or nitride                                    8.  Trench Etch #2                                                                             Cl.sub.2 RIE   Cl.sub.2 CAIBE                                9.  Release      SF.sub.6 RIE   BCl.sub.3 RIE                                 10. Sputter metal                                                                              Aluminum or                                                                   equivalent metal                                             11. Wire bond                                                                 ______________________________________                                    

Steps 1 through 10 of the Table correspond to steps 1 through 10 ofFIG. 1. Two processes are outlined in the Table, one for single crystalsilicon substrates and the other for Gallium Arsenide substrates. Thefollowing description refers first to a process for fabricating singlecrystal silicon.

Referring now to FIG. 1A, in Step 1 of the process, a clean, open areaof bare silicon such as a top surface 8 of a single crystal siliconwafer 10 is provided, on which is deposited a layer 12 of oxide (SIO₂),this layer serving as a mask throughout the remainder of the steps.Although this oxide may be grown in a furnace, this would be suitableonly for discrete elements, for the formation of a thermal oxiderequires temperatures of 1000° or more, which would damage anyelectrical elements on the wafer. The oxide layer 12 can be deposited byPECVD or by some combination of PECVD and thermal growth, but a standardPECVD process is preferred because of its high deposition rate and thefact that it has a low deposition temperature, in the range of 300-500°,which is suitable for fabrication of integral elements on waferscarrying electrical components. The minimum oxide thickness isdetermined by how fast the later RIE steps (to be described) will etchthe oxide, but in general, the oxide mask layer can be a great dealthicker than the required minimum thickness. In a typical process wherethe beam or other structure height is to be 10 μm, a thickness of about0.7 μm is desirable for the oxide.

The height of the structure to be fabricated is, more precisely, theheight of a silicon "skeleton" which will be located inside theoxide-coated released beams fabricated by the present process. Thissilicon skeleton is the major component of the beams produced by theprocess and, as noted above, typically can have a height of about 10 μmor more with a minimum height of about 3 μm.

Step 2, illustrated in FIG. 1B, is a photolithography step wherein aresist material 14 is spun onto the top surface 16 of the mask oxide, isexposed, and is developed, using standard optical photolithographicresist techniques to produce a pattern 18. Optical techniques arepreferred, since a resolution of less than about 0.5 μm generally is notrequired; however, if better resolution is needed, electron beamlithography with a tri-level resist and an aluminum lift-off mask can beused.

Using the pattern 18 developed in the resist, the underlying oxide layer12 is etched to transfer the pattern to the oxide mask, as illustratedin FIG. 1C, Step 3. As there illustrated, the oxide layer 12 is etchedusing, for example, CHF₃ in a magnetron-ion etching (MIE) machine. Suchetching is virtually identical to RIE, except it utilizes a large magnetwhich concentrates the plasma and increases the etch rate. The high etchrate of MIE helps to prevent resist burning. In addition, the MIE etchesat a very low pressure (about 1 to 3 mTorr), provides vertical sidewalls and increases throughput. The pattern 18 formed in the resistmaterial 14 is thus transferred to the mask oxide 12, as illustrated bymask pattern 20.

The resist material is not used to mask the pattern 18 directly onto thesilicon substrate 10 because the Cl₂ etch (to be described), which isused to dig the trenches in the silicon to form the beam structures,etches resist material much more quickly than oxide. Thus, the use of anoxide mask allows the silicon etch to last longer and to dig deeper thancould be done with the same thickness of resist.

Following the pattern transfer of step 3, the resist material isstripped, as illustrated in Step 4, FIG. 1D. The resist is removed inknown manner as, for example, by an O₂ plasma etch utilizing a "barrelasher", wherein ionized oxygen radicals attack the resist, or byutilizing a wet chemical resist strip using a chemical solvent thatdissolves the resist. The O₂ plasma etch is preferred since it is fast,removes the resist completely to leave a clean surface, and has a highthroughput.

The deep silicon etch illustrated in step 5 (FIG. 1E) is critical to theprocess of the present invention. This is a deep silicon etch in whichthe pattern 20 is transferred into the silicon 10 by a deep, verticalanisotropic BCl₃ /Cl₂ RIE to form deep trenches 22. The etch is carriedout in three steps in a standard RIE chamber, as illustrated in thefollowing Table II.

                  TABLE II                                                        ______________________________________                                        Parameter Step 1      Step 2     Step 3                                       ______________________________________                                        Power:    200 Volts   300 Volts  475 Volts                                    Pressure:  20 mTorr    20 mTorr   40 mTorr                                    Time:      1 min.      1 min.    (user defined)                               Chemistry:                                                                    Cl.sub.2   0 sccm      2 sccm     50 sccm                                     BCl.sub.3  14 sccm     14 sccm    5 sccm                                      H.sub.2    7 sccm      7 sccm     0 sccm                                      Etch Rate: Si                                                                           1900Å/min. =                                                                          11.4 μm/hr                                           Etch Rate: SiO.sub.2                                                                    96Å/min. =                                                                            0.576 μm/hr                                          Selectivity:                                                                            20:1:Silicon:Oxide                                                  ______________________________________                                    

Because of the very high selectivity in the main Cl₂ etch, in the rangeof 20 to 1 for silicon and silicon oxide, the presence of a native oxidecan cause unwanted masking. Therefore, the first two etch steps are usedto attack and remove the first 10 to 20 μm of oxide, while the thirdstep is used to carve out the silicon trench. The BCl₃ gas etches oxide,while the Cl₂ etches silicon. A better selectivity in the etch could beobtained by eliminating the BCl₃ and using pure Cl₂ so that 40 μm ofsilicon would be etched for every 1 μm of oxide. However, any smallpieces of oxide that might fall into the trench would act as anundesired mask because of this selectivity. This commonly happens duringetching, with sputtered pieces of oxide falling into the trench andacting as minute masks. The BCl₃ etches the oxide that falls into thetrench and thus improves the quality of etch.

The Cl₂ /BCl₃ etch has an additional effect which is very helpful;namely, in situ side wall thin film deposition. This etch deposits athin layer 24 of silicon dioxide on the side wall and bottom of thetrenches during the etch, and this thin layer acts as an additional sidewall mask which can be used in subsequent steps.

At this point in the process, as illustrated in FIG. 1E, deep trenches22 have been carved into the substrate 10, following the pattern 20 ofthe mask oxide 12. A cross-section of the substrate, such as that shownin FIG. 1E, reveals one or more silicon islands 26 (only one of which isillustrated) surrounded by trenches which separate the islands from thesurrounding substrate which forms a mesa 27. The islands can be anyselected width, for example about one micron, can be several micronstall, can be hundreds of microns long, and can be in any desired shapeor pattern, as established by the photolithograph step. The islands 26and the surrounding substrate 27 may both be referred to as mesas, andeach has an oxide mask 12 on the mesa top and a thin oxide layer 24 onthe side walls caused by the Cl₂ etch. The core of the island 26 issingle crystal silicon, as is the substrate 10 which forms mesa 27.

In order to release the island structures, a protective film is providedon the top surface and on the side walls, with the trench bottom beingbare silicon so that an isotropic RIE can be used to etch the siliconout from under the islands to form cantilevered or free standing beams,and to etch the silicon out from under the surrounding mesa to formundercut cavities.

More particularly, a protective film for the silicon islands andsurrounding mesa is provided in accordance with step 6, illustrated inFIG. 1F, wherein the entire wafer is coated with a protective oxidelayer 28, using a standard plasma enhanced chemical vapor deposition(PECVD) process. This is very similar to an RIE, except that the gaseswhich flow into the chamber react to form films, rather than attackthem. SiH₄ and NO₂ react to form silicon dioxide and NH₃, with theindividual molecules of silicon dioxide bonding to the surface of thewafer and forming a thin film of oxide. The reaction is very conformal;that is, it covers all surfaces with an equally thick layer, regardlessof angle or orientation. The layer of oxide 28 illustrated in FIG. 1Fincorporates the thin layer 24 illustrated in FIG. 1E. In accordancewith this process, the layer 28 extends over the mask layer 12 so thatthe thickness of the silicon dioxide on the island and mesa tops isthicker than that at the floor 30 of the trenches 22.

As illustrated in FIG. 1G, in step 7 of the process the oxide layer 28on the floor 30 of the trenches is removed by an anisotropic CF₄ /O₂RIE. This etch carves straight down, with very little lateral influenceand is easily done with a low pressure RIE. This effectively removes onelayer from every horizontal surface; in this case, the island and mesatop surfaces and the trench bottom, and does not affect the side walls,thereby leaving an oxide layer that covers the top and sides of theisland 26 and surrounding mesa 27, but leaves the floor 30 open, asillustrated in FIG. 1G.

With the trench floor 30 film-free, a second anisotropic Cl₂ /BCl₃ RIEsilicon etch is carried out, as illustrated at step 8 in FIG. 1H. Atthis point, the mask and side wall oxides 12 and 28 are in place on themesas 26 and 27 and only the trench floor is film-free silicon. Thissecond silicon etch adds about 3 to 5 μm of depth to the trench bottom30; that is, the trench bottom is etched 3 to 5 μm below the lower edge32 of the side wall oxide layer 28, thereby exposing 3 to 5 μm of sidewall silicon 34. This etch facilitates the next following isotropicrelease etch (step 9) by exposing the silicon at the bases of theislands 26. This etch has three benefits; it facilitates the release ofthe structures, it lowers the trench floor to control the spacingbetween the beam, when it is released, and the floor, and it preventsexcessive upward etching of the structure behind the side wall oxidecoating 28.

Step 9, FIG. 1I, illustrates the isotropic release etch step whichseparates and releases the islands from the underlying substrate toproduce released beams such as the beam 36, which may, for example, beconnected at one end to extend in a cantilevered fashion over the floorof the trench. This etch step is an SF₆ isotropic high pressure, lowpower RIE process which attacks the silicon at the trench floor 30 andthe lower side wall portion 34 to etch the silicon from under the mesasin the region 38 indicated by the dotted lines. The island 36 is narrowin the region of the etch so it is fully released. The SF₆ etch alsoetches out a cavity 40 in the side wall of the substrate mesa 27,adjacent the location of the beam 36, the upper portion of the mesa sidewall being protected by the oxide layers 12 and 28, discussed above,creating a new trench floor 30'.

It is noted that although the second silicon etch of step 8 ispreferred, it is not essential because the SF₆ release etch of step 9can etch downwardly through the floor 30 of the trench and eventuallywill etch away under the mesas. However, this takes an additional amountof etching time because the SF₆ etchant reaction is "transport limited";that is, the etchant can't reach the surfaces to be etched fast enoughin the narrow trenches. In contrast, the SF₆ etch will be more efficientif step No. 8 is included, for in that case a large, open silicon sidewall is presented which allows lateral etching to begin immediately tominimize the required SF₆ release time.

By including step No. 8, the vertical distance between the releasedbeams 36 and the trench bottom 30 (FIG. 1I) can be controlled. If stepNo. 8 is excluded, the beam to floor distance will depend only on theSF₆ release time, but by adding a silicon etch before the release thisdistance can be increased as required.

It has been found that as the SF₆ etchant begins to etch underneath themesas 24, it begins to reach up underneath the beams, within theprotective layers 28, since the etchant etches in all directionsequally. Normally, a plasma etch is directed toward to the surface to beetched with an accelerating potential which drives the etchant down tothe substrate, so an RIE normally is an inherently directional etch.However, isotropic etching can be obtained by increasing the pressure sothat the directionality is less evident and lateral etching is moreevident, and the present invention preferably utilizes a sufficientlyhigh pressure to make the release more isotropic than directional and torapidly undercut the island.

If the deepening of step 8 is not used, the initial distance from theundersurface of beam 36 to floor 30 will be from 3-5 μm. However, if theetch of step 8 is used, this distance increases to between 5 and 9 μmfrom beam 36 to floor 30' by the end of the etch. The increase in thedistance between the beam and the floor of the trench during the etchingprocess also limits the upward etch rate under the protective oxidelayers 28, thus ensuring a high aspect ratio for the released beam 36.

As illustrated in step 10, FIG. 1J, a sputter deposition of aluminum canbe used to provide metallization of the beam top surface and side walls,as well as the surface and side walls of mesa 27. Such a metal layer isillustrated at 44 in FIG. 1J, with the spacing between the beam and thefloor 30 and the cavity 40 providing isolating breaks in the coating.The metallization on the top surfaces and side walls of the mesas isthus isolated from that on the floor 30 of the trenches, therebyproducing a complete, electrically isolated metallized structure such asthe beam 36 which can then be interconnected to suitable circuitry. Itis noted that the metal layer 44 is also electrically isolated from theunderlying silicon by oxide layers 12 and 28.

FIG. 1K illustrates an alternative to FIG. 1I, wherein the released etchof step 9 is continued for an extended period of time to increase thedepth of the trenches 22 produce an enlarged cavity 40' in mesa 27 andto increase the distance between floor 30 and the released beam 36. Thecavity 40' extends upwardly behind the side wall oxide layer 28 so thatwhen the metal layer 44 is applied (step 10) an extended leakage path,indicated by dotted line 46 is created to reduce leakage between metal44 and the substrate silicon in the mesa 27. If desired, the etch can becontinued through the bottom of the substrate to produce an aperturethrough the wafer in which the MEM structure such as beam 36 issuspended.

Extension of the etching time also serves to etch the bottom of beam 36,producing a cavity 48 between the beam side wall oxide and metal layers,again as illustrated in FIG. 1K. The reduced side of beam 36 reduces itsmass and increases its flexibility, a feature which is highly desirablein some applications.

FIG. 1L illustrates a further alternative to the process described inFIGS. 1A-1J, wherein a conformal dielectric layer 49 of CVD oxide ornitride is deposited on the structure of FIG. 11 before themetallization step of FIG. 1J. This conformal layer covers the top,bottom and sides of released beam 36, covers the sidewall oxide 28 onthe mesa 27, and covers the floor 30' of the etched trenches, providingan insulating layer between the underlying substrate and themetallization layer 44 deposited in accordance with FIG. 1J.

A further alternative is to thermally grow oxide on the exposed siliconsurfaces (floor 30' and the bottom of beam 36) of FIG. 1I to provideinsulation between the metal layer 44 and the silicon substrate.However, this is less preferred, since thermal oxide is a hightemperature process. Further, the deposition process of FIG. 1L providesa thicker layer of dielectric on the mesas, and increases the electricalpath between the metal layer on the sidewalls and the metal on floor30'.

The foregoing description of the process set out in steps 1 through 10of Table I is directed to the application of the process to singlecrystal silicon. However, the process is also applicable to othermaterials, such as gallium arsenide, as set out in the correspondingright-hand column in Table I. These steps are also illustrated in FIGS.1A through 1J, although different materials are used, as set out in theTable. Thus, for example, in step 1 the dielectric mask is a PECVD oxideor nitride, the oxide being deposited since the gallium arsenide cannotbe oxidized in the manner of silicon. The dielectric may also benitride, if desired.

The photolithography process of step 2 is the same, as is the patterntransfer of step 3. When using a gallium arsenide substrate, the resistremoval process of step 4 is optional, and may not be required.

The trench etch of step 5 is a CAIBE process using Cl₂, rather than theRIE process used with single crystal silicon. The side wall dielectricis applied by PECVD oxide or nitride, and step 7 utilizes CF₄ RIE in themanner used with single crystal silicon.

The trench etch step 8 is again a Cl₂ CAIBE process, while the releasestep 9 uses BCl₃ RIE. With these changes, the process of the presentinvention can be used to produce gallium arsenide released beamstructures which can be metalized in accordance with step 10 in a singlemask process.

If the released structure is in a discrete wafer, the device may beconnected as by a wire bond (Step 11 in Table I) to suitable externalcircuitry. A suitable structure for such wire bonding is illustrated inFIG. 2 which is a top view of a simple microelectromechanical deviceconstructed in accordance with the process of FIG. 1. As thereillustrated, a wafer 50 of single crystal silicon is patterned toproduce a single released beam 52 located in a trench 54, the beam 52being similar to beam 36 of FIG. 1J. Formed on each side of the releasedbeam 52 by trench 54 are fixed structures such as electrodes 56 and 58,illustrated in cross-section in FIG. 3 and corresponding to mesa 27 inFIG. 1J. The cross-section is taken along lines 3--3 of FIG. 2.

Also defined by the trench 54 are additional fixed structures such ascontact pads 60, 62 and 64 and their corresponding interconnects. Pad 62is connected to the released beam 52 by way of interconnect 66, asillustrated in FIG. 4, which is a cross-section taken along line 4--4 ofFIG. 2. The interconnect is illustrated as being wider than the releasedbeam 52 of FIG. 3, and is not released from the substrate 50 so that itis stationary and provides a support wall at region 68 (FIG. 2) formounting a fixed end of the cantilevered released beam 52. Contact pads60 and 64 are connected to the electrodes 56 and 58, respectively, byway of interconnects 70 and 72, respectively, these interconnects beingsimilar to interconnect 66. As illustrated, the interconnects, contactpads, and electrode supports are all formed as mesas, are covered bydielectric layers 74 and 76 on the top surfaces and side walls:respectively and are surrounded by the trench 54. Upon metallization ofthe device, as described with respect to FIG. 1J, a metal coating 78will be applied between the contact pad 62 and beam 52 only by way ofinterconnect 66, will connect contact pad 60 to electrode 56 only by wayof interconnect 70, and will connect contact pad 64 to electrode 58 onlyby way of interconnect 72. Thus, the trench 54 and the oxide layers 74and 76 electrically isolate the metal from the substrate and fromadjacent structures.

The metal layer 78 on the electrodes 56 and 58 includes vertical sidewall portions which cooperate with the side wall portions of the metallayer 78 on released beam 52 to produce the parallel plates ofcapacitors. When the two electrodes 56 and 58 are located in closeproximity to the beam 52, the capacitance therebetween, which is afunction of distance, can be measured. Therefore, any movement of thebeam 52 with respect to beams 56 and 58 can be accurately measured. Theside wall parallel plates can be in the range of 10 to 20 μm tall,depending on the height of the beam 52, and can have 1 to 2 μm ofinterplate spacing. The beam and electrodes can be several thousands ofmicrons in length, so that a large capacitance can be provided. Such astructure has an important use as an accelerometer by measuring relativemotion of the released beam with respect to the stationary electrodes.

It is noted that the process of the present invention permits automaticdefinition of the metallization for the capacitors, interconnects, andcontact pads without the need for additional photolithographic steps.All that is required is that the shape of these structures beestablished by the initial oxide layer which defines the location of thetrenches, and these trenches then serve to isolate the metal from thesubstrate, as described above with respect to FIGS. 1A-1K. Theoverhanging oxide produced by the cavity 40 illustrated in step 9 (FIGS.1I or 1K) prevents the sputtered metal from contacting the substrate andprevents leakage current between the metal and the substrate to ensurethis isolation. The sputter deposition of the metal is semiconformal andwill laterally deposit on the side walls, but will not depositunderneath the overhanging edges of the oxide. Accordingly, the processprovides a self-aligned and self-defined movable microelectromechanicaldevice and provides electrical connectors for that device.

In addition to fabrication of the discrete structure illustrated inFIGS. 2-4, the process of the present invention permits fabrication ofintegrated structures, wherein released beams and othermicroelectromechanical structures may be interconnected with an on-chipintegrated circuit (IC). In accordance with this modification of theprocess, which is illustrated in FIGS. 5A-5I, an integrated circuitwafer can be batch fabricated using standard integrated circuittechnologies to produce desired circuits and circuit components on thewafer, and thereafter a microelectromechanical device can be fabricatedon the wafer utilizing the process of the present invention withoutvarying the standard IC technology. In this embodiment, the singlecrystal silicon process described above is utilized, since IC wafers areconventionally silicon. The single crystal silicon process of theinvention operates at a sufficiently low temperature to permit thefabrication of released structures on existing IC wafers without thermaldamage to the existing circuitry or circuit components. Of course, sucha wafer could incorporate a segment of another substrate material suchas gallium arsenide, if desired, in which case the alternative processof the present invention could be used, again without thermal damage tothe existing circuits.

The process of fabricating microelectromechanical devices on existing ICwafers is outlined in the following Table III.

                  TABLE III                                                       ______________________________________                                        Step Number      Description                                                  ______________________________________                                        0a.     Photolithography                                                                           Thin Passivation region for                                                   MEM device                                                                    Vapor prime; spin, bake,                                                      expose, develop & descum                                 0b.     CF.sub.4 isotropic RIE                                                                     Thin passivation layer                                   0c.     O.sub.2 plasma etch                                                                        Remove residual resist                                   Steps 1-9            (Same as Table I)                                        10.     Photolithography                                                                           Open via windows to buried                                                    contacts                                                                      Vapor prime; spin, bake,                                                      expose, develop & descum                                 11.     CF.sub.4 Isotropic RIE                                                                     Etch oxide                                               12.     Wet chemical resist                                                                        Remove residual resist                                           strip & O.sub.2 plasma                                                13.     Sputter metal                                                                              Aluminum (pumpdown & deposit)                            14.     Photolithography                                                                           Pattern top level metal                                                       Vapor prime; spin, bake,                                                      long exposure, long                                                           development & descum                                     15.     Cl.sub.2 Isotropic RIE                                                                     Etch sidewall aluminum                                   16.     Wet chemical resist                                                                        Remove residual resist                                           strip & O.sub.2 plasma                                                17      PECVD Dielectric                                                                           Deposit thin passivation                                                      dielectric                                               ______________________________________                                    

In order to integrate released beams fabricated in accordance with thepresent invention, it is necessary that the fabricated integratedcircuit die, or wafer, incorporate one or more buried metal contactpads, have a thick passivation oxide layer, and have sufficient spaceavailable for the microelectromechanical (MEM) device. Any kind ofintegrated circuit can be on the wafer for connection to the releasedstructure as long as any connections to the circuit or circuitcomponents are by way of metal pads. These pads are buried directlybeneath a passivation oxide layer so that they can be accessed by way ofa via etch. The passivation layer preferably is PECVD oxide across theentire wafer, and is used as the mask oxide for the fabrication processof the present invention. Such a passivation oxide is conventionallyplaced over a finished IC wafer as a hermetic seal against water andother contaminants during handling and thus provides a convenientbeginning point for the present process. This layer is generally in therange of 0.5 to 1.0 μm thick, but if this is too thin, additional PECVDdielectric material such as oxide or nitride can be added in thefabrication process of the present invention. On the other hand, if itis too thick, or does not provide a good enough mask, then it can beremoved and replaced by another PECVD oxide or nitride layer as needed.Preparation of the IC wafer prior to the fabrication of the MEM isdescribed in steps a, b, and c, outlined in Table III, above, and isillustrated in FIGS. 5A through 5C, to which reference is now made.

An integrated circuit (IC) wafer 100 includes a passivation oxide layer102 which is formed as a final step in conventional IC fabrication toserve as a protective layer over the wafer. The thickness of this oxidelayer preferably is no less than the minimum mask requirements discussedabove. The wafer includes, for example, a metal contact pad 104 locatedin an insulating layer 105 such as oxide, the pad being a part of anintegrated circuit (not shown) and connected into that circuit in knownmanner. The wafer also includes a region generally indicated at 106 forreceiving a MEM device. In general, the MEM device region will include asecond layer 108 of oxide in addition to the passivation layer 102. Thissecond layer is a field oxide, which is a thermally grown wet oxideplaced between active devices as an isolation layer. The passivationoxide 102 is generally 0.5 to 1.0 μm thick, while the field oxide 108 isgenerally 0.6 μm thick.

Step 0a in FIG. 5A illustrates a photolithographic step which preparesthe passivation layer 102 for fabrication of the MEM device. A resistlayer 110 is spun onto the top surface of the wafer, is exposed and isdeveloped to produce a pattern 112 defining the location of the MEMdevice. Thereafter, the oxide layers 102 and 108 can be thinned, ifrequired, at step 0b in FIG. 5B, as by CF₄ isotropic RIE, leaving only apart of layer 102, as illustrated. Alternatively, the oxide can bethickened by the addition of more PECVD oxide, if required. The residualresist layer 110 is then removed by an O₂ plasma etch, as illustrated inFIG. 5C. In this illustrated embodiment, the photolithographic processthins the passivation oxide in the region 106 set aside for the MEMdevices, without thinning the oxide over the previously fabricated ICdevices, allowing more precise control over the mask oxide thickness.

Following the preparatory treatment of the wafer in steps 5A through 5C,the MEM device is processed within the region 106 of wafer 100,following steps 1-9 illustrated in FIGS. 1A through 1I, resulting in thedielectric-covered structure of FIG. 5D (also labelled as step 9), priorto metallization. Thereafter, the process of the present embodimentincludes opening a via window to the buried contact and connecting it tothe MEM device. As described in step 10 in Table III, a resist 120 (FIG.5E) is spun onto the top surface of the mask oxide layer 122, whichconsists of the passivation layer 102, as well as the additional oxidelayers on the top surface and on the trench side walls that are addedduring the fabrication of the released structure in steps 1-9. Theresist 120 is exposed and developed to produce a via mask opening 124 inalignment with the contact pad 104. The oxide layer 122 is etchedthrough the via mask opening 124 as by an isotropic CF₄ RIE illustratedin FIG. 5F, to produce via 126. Thereafter, the resist is removed by awet chemical and O₂ plasma stripping step, also illustrated in FIG. 5Fas step 12, thereby exposing the contact pad through a via 126.

Step 13 (FIG. 5G) is a sputter deposition step in which a metal layer130 is deposited on the top surface of the wafer 100 and on the floorand the side walls of the trenches 54 in the manner illustrated anddescribed with respect to FIG. 1J.

The deposited metal 130 is conformal, and coats contact pad 104 tothereby interconnect the MEM structure (for example, beam 52) with thecontact 104. It will be understood that other similar contacts may alsobe connected to the metal layer by similar vias. Thereafter, the metal130 is trimmed back to define the specific shape of the interconnectsbetween the IC contact points and the MEM structure. Since the sputterdeposition is conformal and deposits metal on the top surfaces, down theside walls and on the floor of all the trenches, as illustrated in FIG.5G, it becomes necessary to pattern the metal to limit the metallizationto the desired interconnections. This involves photolithography acrossmesa tops, side walls, and trench-bottom surfaces, and is accomplishedby first applying a high viscosity resist 132 to planarize the wafer,the resist filling the trenches and protectively coating the MEMdevices. A long exposure is used to pattern the thick resist material,so that the patterning can be carried out through as much as 20micrometers of resist material. A long development time is then requiredto develop the exposed resist.

FIG. 5H illustrates the resist material 132 which remains after exposureand development in step 14 of the process. Although resolution is lostby using a thick resist, long exposure times and long development times,nevertheless the resolution obtained by this process is more thansufficient. Although to a large extent the metal layer 130 is etched bythe development process, nevertheless it is preferred that following thedevelopment of the resist layer 132, an isotropic Cl₂ RIE is carried outto isotropically etch all exposed aluminum. Thus, the aluminum notcovered by resist 132 is etched away, as illustrated in FIG. 5I at step15, leaving the metal layer 130 only in the via 126, along the locationof the desired interconnect 134, and onto the cantilevered beam 52, soas to connect the beam to the integrated circuit. The resist layer 132is then removed by the wet chemical and O₂ plasma resist strip of step16, also in FIG. 5I.

As a final step, a thin conformal PECVD dielectric is deposited on theentire surface of the wafer to insulate and passivate the devices. Notethat because MEM structures are designed to move, they cannot be buriedunder several microns of passivation, so only about 100 to 300 nm ofoxide is used for this purpose. Nitride could also be used, although itis stiffer than oxide and may inhibit motion. The oxide deposition ofstep 17 is illustrated by arrows 136 in FIG. 5I.

It will be noted that the interconnection of the MEM device to anintegrated circuit requires additional masking steps, as illustrated inFIGS. 5E through 5I. However, the basic process for forming the MEMstructure is a single mask process, as described above, and it is onlythe later processing that require these additional masking steps.

A further embodiment of the invention is illustrated in FIG. 6, whereinan elongated, released structure such as beam 52 further is treated toincrease its lateral flexibility. The beam, which is fabricated inaccordance with the single-mask process of FIG. 1, includes a metallayer 44 over oxide layers 12 and 28, and these layers tend to reducethe flexibility of the single crystal silicon core 36 of beam 52.Flexibility of the beam can be increased by removing the side wall oxide28 and the side wall metal 44 from the beam in selected locations. Sincethe spring constant varies with the cube of the width of the beam, suchremoval can reduce the spring constant by one to two orders ofmagnitude.

In accordance with this embodiment, a thick resist layer (not shown) isspun onto the wafer, filling in the trench around the released beam. Theresist is exposed in a conventional photolithographic step and developedto expose the aluminum layer 44 in the region 140 illustrated by thedotted lines in FIG. 6. By a Cl₂ isotropic RIE step, the exposedaluminum is etched on the top and side walls of beam 52 in the region140 and the residual resist material is removed by a wet chemical resiststripping step. Thereafter, a layer 142 of aluminum is evaporated onlyonto the top surface of the beam 52 and this evaporated aluminumtogether with the remaining metal layer 44 is used as an etch mask for aCF₄ isotropic RIE etch of the side wall oxide 28 in the regions 144 ofthe beam so that the beams are film-free silicon on the sides in theregion 140. Accordingly, in this region the beam is as narrow as theoriginal silicon island 36, while the top of the beam carries a thinfilm of mask oxide 12 and the evaporated aluminum 142 so that electricalconnectivity is achieved from the contact pads to the MEM device alongthe top surface of spring portion 140 of the beam.

An important use for the released beam 52 fabricated in accordance withthe present invention is as an accelerometer, wherein flexing of thebeam with respect to the capacitive electrodes, as illustrated in FIG.2, is a measure of acceleration applied to the wafer on which the beamis mounted. In order to increase the sensitivity of such an accelerator,the beam can be made in a variety of shapes to increase its mass, forexample, by adding a grid-like structure 150 to the free end of beam 52,as illustrated in FIG. 7. This grid-like structure is fabricatedfollowing the process of FIG. 1, steps 1-9, with the grid shape beingdetermined by the initial photolithographic step. Such a grid may be ofany desired width and length, and with any desired number of beams, andadds mass to the beam 52 to improve its function as an accelerometer. Inlarge designs the grid may be 200 μm by 900 μm, with beams on 5 μm or 10μm grid points. The beams may be from 1 to 2 μm wide with openingsbetween them being 3 to 4 μm or 8 to 9 μm wide. The grid is illustratedas being connected to one end of a released beam 52 which iscantilevered above the floor 30 of the surrounding trench and is mountedto an end wall 152 of a surrounding mesa portion 154. However, it willbe understood that the grid can be placed at the center of an elongatedbeam 52 which can be connected at both ends to end walls of thesurrounding trench 54, the exact shape and size of the grid and itssupports being dependent on the degree of relative movement desired, andthus on the desired sensitivity of the accelerometer. It will beunderstood that a plurality of such movable structures can be providedin a single trench 54 or in a number of trenches on a single wafer, witheach having selective characteristics which may be slightly differentfrom each other to provide a wide range of sensitivity for theaccelerometer device. As illustrated in FIG. 7, the beam 52 in grid 150are covered by a metal layer such as the 10 layer 44 provided in theprocess of FIG. 1 so that the metal on the side wall 156 of the gridcooperates with the metal on the side wall 158 of the mesa 154 toprovide the opposed plates of a capacitor by means of which the motionof the grid with respect to the mesa can be detected or, conversely, bythe application of a potential, the grid can be moved.

The mass of the accelerometer of FIG. 7 can be further increased by theaddition of a layer of tungsten about 5 to 10 μm thick over the surfaceof the grid. The process for adding such a layer of tungsten is outlinedin the following Table IV:

                  TABLE IV                                                        ______________________________________                                        Step Number        Description                                                ______________________________________                                        Steps 1-9              Same as Table 1                                        10.       PECVD Oxide  Deposit thin dielectric                                                       coating to prevent WSCVD                               11.       PECVD Silicon                                                                              Deposit seed layer for                                                        WSCVD                                                  12.       Photolithography                                                                           Pattern seed layer with                                                       thick resist                                                                  Vapor prime; spin, bake,                                                      long exposure, long                                                           development & descum                                   13.       SF.sub.6 Isotropic                                                                         Etch seed layer                                                  RIE of aSi                                                          14.       Wet chemical resist                                                                        Remove residual resist                                           strip & O.sub.2 plasma                                              15.       WSCVD        Tungsten Selective Chemical                                                   Vapor Deposition                                       16.       Photolithography                                                                           Trim undesired tungsten with                                                  thick resist                                                                  Vapor prime; spin, bake,                                                      long exposure, long                                                           development; & descum                                  17.       Wet Chemical of W                                                                          Etch unwanted Tungsten                                 18.       Wet Chemical Remove residual resist                                           resist strip &                                                                O.sub.2 plasma                                                      19.       Metal Sputter                                                                              Same as FIG. 2                                                   Deposition                                                                    (e.g. Al)                                                           20.       Wire Bond    Same as FIG. 2                                         ______________________________________                                    

As illustrated in Table IV and in FIG. 8A, the process of applying atungsten layer is initiated after the release of the beams at step 9illustrated in FIG. 1I. At that point in the process, the silicon beams52 are covered on the top and side by oxide layers 12 and 28 and thebeams are cantilevered over the floor 30 of deep silicon trenches 54.The undersides of the beams and the trench floors are film-free silicon.Since tungsten does not deposit on nonmetal surfaces, an attempt toapply tungsten to the structure at this point in the process wouldresult in the tungsten growing only on the trench floor and on the beamundersides. To prevent that, a conformal dielectric coating such asPECVD oxide 160 (FIG. 8B) is deposited on the entire wafer. This layer160 services as an etch stop for the seed layer patterning step, andprevents the tungsten from contacting the substrate underneath the beamsand on the trench floor.

With the wafer lightly sealed off by the dielectric layer 160, a thinseed layer 162 of amorphous silicon (FIG. 8C, Step 11 of Table IV) isthen deposited on the wafer, covering all surfaces. Because of theunderlying dielectric layer 160, the silicon is electrically isolatedfrom the substrate. A thick photoresist layer 164 is then applied to thewafer, filling the trench 54 and covering the grid 150 as well as thetop of the mesa 154. The resist 164 is then exposed and developed, asillustrated in FIG. 8E, which is a continuation of step 12, to removethe photoresist material from areas where tungsten is not desired. Thus,the photoresist material is removed from the mesa 154 and from one ofthe grid beams 52, the figure showing a beam 166 freed of thephotoresist layer.

In FIG. 8E, removal of the seed layer 162 from the exposed beam 166 andfrom the exposed mesa 154 is illustrated. An isotropic SF₆ RIE removesthe exposed layer 162 of silicon, with the oxide underlayer 160 servingas an etch stop. This exposes the dielectric layer 162 on beam 166 andmesa 154, as illustrated in FIG. 8F to prevent the growth of tungsten inthese areas.

As illustrated in FIG. 8G, step 14 of Table IV, the resist 164 is nextremoved from the wafer to expose the grid 150, leaving the silicon layer162 exposed on the beams indicated at 168 and 170 and on a portion ofthe floor 30 of trench 54. Thereafter, in step 15, illustrated in FIG.8H, a tungsten selective CVD (WSCVD) is deposited, with deposition onlyoccurring on the exposed seed layer 162 and being prevented on the oxide160, thereby producing a tungsten layer 170 which is several micronsthick on both the grid 150 and on the floor 30 where silicon is exposed.By depositing a thick layer of tungsten, a "blanket" is formed on top ofthe grid 150 and extends across the gaps between adjacent beams, asillustrated between beams 168 and 170 in FIG. 8H.

Since the process of tungsten deposition is not perfect, some tungstenwill be deposited on the oxide layers 160, and therefore the samephotolithographic process that was used to define the area of thetungsten is used to re-expose the areas around the grid 150 and a quickwet-chemical tungsten etch is used to clean the oxide surfaces of thespurious tungsten. Thus, as illustrated in FIG. 8I, step 16, aphotoresist layer 170 is applied to the wafer, again filling the trench54 in the region of the mesa 154 and the beam 166. The resist 172 isexposed and developed as illustrated in step 17 to uncover the oxidelayer 160 so that a wet chemical etch of the unwanted tungsten can beperformed in those regions. The resist 172 is then stripped (FIG. 8K),leaving a blanket of tungsten 170 across the selected part of the grid150, as illustrated. The blanket of tungsten covers a selected portionof the top of the grid and fills in the interbeam spaces to produce thethick conformal blanket.

After the tungsten has been deposited on the MEM device as describedabove, the remainder of the device is covered by the oxide coating 160,and accordingly, the standard SCREAM-I process can be continued by asputter deposition of metal such as aluminum, in accordance with step 10of FIG. 1J, to produce a metal coating 180 on the opposed sidewalls ofgrid 150 and mesa 154, as illustrated in FIG. 8L, and as discussed abovewith respect to FIG. 7.

As indicated above, the process of FIG. 1A through 1I can be used tofabricated a wide variety of structures. A grid structure has beendiscussed above with respect to FIG. 7, and FIG. 9, to which referenceis now made, illustrates another example of such a grid structurefabricated using the process of FIGS. 1A through 1I. In this case, acomplex grid 200 is fabricated within a trench 202 and is securedbetween opposed trench walls 204 and 206 by means of axially alignedbeams 208 and 210. The grid includes a plurality of longitudinal beamssuch as beams 212 and a plurality of cross beams such as beams 214.These beams are spaced apart to define spaced openings or apertures 216.

The grid 200 is fabricated for torsional motion, with the mounting beams208 and 210 serving as micron-size torsion rods and the entire devicebeing fabricated from single crystal silicon, as discussed above. Asdescribed with respect to FIGS. 1A through 1I, a sequence of dielectricdepositions and reactive ion etches are used to define and release thesilicon beams 208, 210, 212 and 214 from the substrate silicon 218. Toproduce torsional motion, an electrode actuator scheme is provided inaccordance with either FIGS. 10A through 10C or FIGS. 11A through 11C,both sets of figures being cross sectional views taken at 10--10 of FIG.9. The processes illustrated in FIGS. 10A through 10C and FIGS. 11Athrough 11C differ in how the metal electrodes used in actuating thetorsional resonator 200 are defined. In both cases, the electrodes arefabricated by an isotropic dry etch and the released structure is formedby an undercut-etch of the silicon beams. The grid 200 serves toincrease the mass of the resonator, as discussed above, with respect toFIG. 7. When suitable potentials are applied across these electrodes,the resonator 200 rotates about the common axis of beams 208 and 210, asindicated by the arrows 220. The torsion beams 208 and 210 may be about0.8 μm wide and 2 μm high, for example.

In accordance with the process of FIGS. 10A through 10C, the trench 202is etched in the substrate 218 with the surfaces covered by a mask oxide222, in the manner illustrated in FIG. 1F. The bottom of the trench 202is etched in the manner in FIGS. 1G and 1H, and the beams 212 andreleased in the manner described with respect to FIG. 1I, leaving thebeams spaced above the floor 224 of the trench. Thereafter, asillustrated in FIG. 10C, metal 226 is evaporated through the releasedstructure to coat the top surface and side walls of the surrounding mesaand the beams 212, as previously discussed, and also providing metal onthe bottom wall 224, as illustrated at 228 and 230 in FIG. 10C.Shadowing of the evaporated metal by the grid structure 200, and moreparticularly by beams 212 in FIG. 10C, produces a break 232 in the metalon floor 224 of the trench so that the metal area 228 lying between thegrid 200 and side wall 234 of trench 202 (FIG. 9) is electricallyisolated from the remaining metal on the floor of the trench. Byextending the trench past the end wall 206 as indicated at 236 to acontact pad diagrammatically illustrated at 238, electrical connectionscan be made to the metal 228 to form an electrode. Similarly, the metal226 on the grid 200 can also be electrically connected to a suitablepad, in the manner illustrated in FIG. 2, so that a potential can beapplied between the metal on the grid 200 and the metal layer 228 on thefloor of the trench.

A similar electrode structure is provided on the opposite side of grid200, adjacent the side wall 240 (FIG. 9) with the electrode formed onthe floor of the trench adjacent wall 240 extending to a contact pad 242for connection to suitable circuitry. Potentials provided between thegrid 200 and either of the electrode adjacent wall 234 or the electrodeadjacent wall 240 apply torsional forces to the grid 200, causing it topivot about its mounting beams 208, 210, so that the grid twists in thedirection of arrows 220, raising one lateral edge or the other out ofthe plane of substrate 218.

FIGS. 11A through 11C provide an alternative electrode arrangement, butin this case a second masking step is required. As illustrated in FIG.11A, the wafer is patterned to form islands 250 covered by an oxidelayer 252 in the manner illustrated in FIG. 1F. A metal layer 254 isthen applied to the entire top surface of the wafer and the metal ispatterned to remove it from the floor of the trenches between theislands 250 and to partially remove the metal from the floor of thetrench extending between the islands and the substrate side wall 256.Thereafter, the oxide layer 252 is removed from the floors of thetrenches in the manner illustrated in FIG. 1G, and the silicon is etchedin the manner described with respect to FIG. 11 to release the grid 200,and its corresponding beams 212 (FIG. 11C). This process leaves a sidewall electrode on wall 256 with a horizontal lip 258 which extendstoward the grid 200 and functions generally in the manner of the bottomelectrode 228 described with respect to FIG. 10C.

In both of the structures of FIGS. 10C and 11C, the metal quoting on thegrid is capacitively coupled with the corresponding electrode 228 or258, respectively, to provide the necessary torsional force on grid 200.No additional external electrodes above the surface of the wafer arerequired to produce rotational motion out of the plane of the substrate.If desired, the grid 200 dam be covered tungsten in the manner describedwith respect to FIGS. 8A through 8L to increase its mass. Furthermore,the tungsten can be mechanically polished to a flat surface so that themetal layer can act as a mirror, whereby rotation of the grid about theaxes 208, 210 can provide accurate directional control of a reflectedbeam of light. Such a structure is illustrated in FIG. 12 wherein atungsten layer 260 is shown as having a flat, polished top surface 262.

It is noted that the capacitive effect between the stationary electrodeson the substrate and the movable electrodes on the grid in the devicesof FIG. 10C and 11C are dependent upon the spacing between the grids aswell as upon the potential applied between the electrodes. In accordancewith the process of the present invention, this spacing can be in therange of 5 μm so that this capacitance is more significant than anycapacitance that exists between the grid and the ground plane of thesubstrate beneath the grid, thereby allowing the potential across thecapacitive plates to control the motion of the grid.

Although tungsten is preferred as the coating for the grid, asillustrated in FIGS. 8L and 12, it will be understood that a dielectricsuch as nitride or oxide can be used as a membrane on a grid structuresuch as that illustrated in FIG. 7, or even on the structure of FIG. 9,for use in pressure sensing or like applications.

Although the foregoing illustrations have all been directed toessentially rectangular beam structures, it will be understood that thephotolithographic process described above can be used to fabricate anydesired shape. Examples of such shapes are illustrated in FIG. 13,wherein a pair of spiral beams 270 and 272 are fabricated in a trench274 formed in a substrate, or wafer 276. The spiral shape of the beamstructures is produced in the photolithographic step, and the resultingbeam is coated by a dielectric and by a metal layer in the mannerillustrated in FIGS. 1A through 1J. One end of the spiral is connectedto a side wall 278 of the cavity 274 and the metal is connected to acontact pad diagrammatically illustrated at 280 on the substrate 276. Insimilar manner, one end of the second spiral 272 is connected to wall78, for example, with its metal layer connected to a contact pad 282.

To connect the opposite end of the spiral to an electrode so that eachspiral can form an inductor, the free end 284 and 286 of spirals 270 and272, respectively, are connected by way of dielectric bridges 288 and290, respectively, to the opposite side wall 292, for example, of trench274. This dielectric bridge extends across the intervening spiralportions of the respective beams to provide supports and electricalisolation for subsequent deposition of electrodes 294 and 26,respectively, which extends to pads at the edge of the wafer forelectrical connection to the spiral beams. In this way,microelectromechanical inductors can be fabricated in accordance withthe invention, with the adjacent inductors 270 and 272 forming amicrotransformer, if desired. If desired, metal electrodes can bedeposited on the floor of the trench 274 beneath one or both of thespiral beams so that a potential can be applied between the electrodeson the beams and on the floor to provide capacitive control of verticalmotion of the beam. Such a variation in the position of one spiral withrespect to the other changes the inductive relationship between the twoto thereby provide a variable transformer. Similarly, electrodes on theside walls of the trench 274, such as side wall 278, permit a capacitiveconnection with electrodes on the side walls of the spirals 270 and 272to permit lateral motion of the spirals to thereby change the spacingbetween adjacent coils and change the inductance of the beams.

In summary, the present invention is directed to a silicon process forfabricating single crystal silicon MEM devices utilizing a single maskto define all components. The process is low temperature, self-aligned,and independent of crystal orientation, and relies on industry standardfabrication processes and tools. The process can be used to producediscrete devices on wafers, or can be used in combination with VLSIintegrated circuit processes without major modification of the ICprocesses because of its low temperature requirements, simple designrules, and short process times. The low temperatures used in the presentprocess do not adversely affect existing IC circuits, so MEM devices canbe fabricated on completed IC wafers. The device as an importantapplication has an accelerometer, allowing such devices to beincorporated in integrated circuits, for example, or fabricated ondiscrete wafers for use in a wide range of applications, with a tungstenprocess being provided to increase the mask of the MEM device for suchapplications.

Although the invention has been described in terms of preferredembodiments, it will be apparent to those of skill in the art thatnumerous variations and modifications may be made without departing fromthe true spirit and scope thereof, as set forth in the following claims:

What is claimed is:
 1. A microelectromechanical structure fabricated ina single crystal substrate independently of crystal orientation by a lowtemperature, single mask process, comprising:a single crystal waferhaving a top surface; trench means in the surface of said wafer defininga released beam and a support for said beam spaced from a surroundingsubstrate, said beam being movable with respect to said substrate; anelectrically insulating layer on said released beam, said support, andsaid substrate; and an electrically conductive coating on saidinsulating layer on said released beam, support and substrate, saidtrench means electrically isolating the conductive coating on said beamfrom the conductive coating on said substrate.
 2. The device of claim 1,further including means for producing a potential difference betweensaid beam coating and said substrate coating for producing motion ofsaid beam in the plane of said top surface.
 3. The device of claim 1,wherein said support for said beam includes contact pad means andinterconnect means including said electrically insulating layer and saidconductive coating extending between said beam and said contact padmeans, said contact pad and interconnect means being defined by saidtrench means.
 4. The device of claim 3, wherein the electricallyconductive coating on said interconnect means and on said contact padmeans is electrically isolated from said substrate conductive coating bysaid trench means.
 5. The device of claim 3, further including electrodemeans and corresponding electrode contact pad and electrode interconnectmeans in the top surface of said wafer, said electrode means having anelectrically conductive coating and being spaced from and electricallyisolated from said beam means by said trench means to provide capacitivecoupling between said released beam and said electrode means.
 6. Thedevice of claim 3, wherein said released beam comprises an elongated,cantilevered arm connected at one end to said support.
 7. The device ofclaim 6, wherein said released beam further comprises enlarged gridmeans carried by said arm.
 8. The device of claim 7, further includingmeans carried by said grid for increasing the mass thereof.
 9. Thedevice of claim 7, further including a membrane carried by said grid.10. The device of claim 7, further including electrode means in saidtrench adjacent at least one edge of said grid and positioned to providecapacitive coupling between said grid and said electrode means toproduce torsional motion of said grid about said cantilevered arm.
 11. Amicromechanical device, comprising:a substrate having a top surface; atrench extending into said substrate through said surface to form spacedwalls and a floor in said substrate, said trench walls each having anundercut portion adjacent said floor; a layer of conductive material onthe surface of said substrate, on said walls, and on said floor, saidundercut portions electrically isolating the layer of conductivematerial on said walls of said trench from the layer of conductivematerial on the floor of said trench.
 12. The device claim 11, whereinsaid trench walls define a structure within a cavity in said substrate,a first of said spaced walls being located on said substrate and asecond of said spaced walls being located on said structure, said trenchfloor extending between said walls and forming a floor for said cavity.13. The device of claim 12, wherein at least a portion of said structureis released from said substrate and is movable with respect to saidsubstrate.
 14. The device of claim 13, wherein at least a portion ofsaid structure is fixed to said substrate, said movable portion of saidstructure having a first end mounted on said fixed portion of saidstructure and a second end spaced from and movable with respect to saidsubstrate.
 15. The device of claim 12, wherein said structure includes atop surface, and further including a conductive material on at least aportion of said top surface and electrically connected to conductivematerial on said second spaced wall.
 16. The device of claim 15, whereinconductive material on said surface of said substrate is electricallyconnected to conductive material on said first spaced wall, saidundercut portions electrically isolating said conductive material onsaid first spaced wall from said conductive material on said secondspaced wall.
 17. The device of claim 15, wherein said first and secondtrench walls are substantially perpendicular to said substrate surface,said second wall forming sides of said structure.
 18. The device ofclaim 17, wherein said trench surrounds said structure, said second wallbeing continuous and surrounding said structure to form an island spacedon all sides from said substrate.
 19. The device of claim 17, whereinsaid trench extends substantially completely around said structure, saidsecond wall being continuous to form a peninsula within said cavity. 20.The device of claim 17, wherein said undercut portions of said spacedwalls extend laterally from trench sufficiently far to produce adiscontinuity in said conductive material on said walls.
 21. Amicrostructure, comprising:a single crystal silicon substrate having atop surface and a trench extending into the substrate through saidsurface to form in the substrate at least a first vertical wall having atop edge at said surface and a base portion intersecting a horizontalfloor; an undercut cavity portion extending into said substrate alongthe intersection of said base portion and said floor to undercut thevertical wall; an electrically insulating layer on said top surface andsaid vertical wall, and a metal layer on the insulating layers on saidtop surface, said vertical wall, and said horizontal floor, the metallayer on said vertical wall being electrically isolated from the metallayer on said floor by said undercut cavity portion.
 22. Themicrostructure of claim 21, further including a second vertical wallspaced from said first wall and having a base portion intersecting saidfloor and a cavity portion extending into said substrate along theintersection of said base portion and said floor to undercut said secondwall.
 23. The microstructure of claim 22, further including aninsulating layer on said second wall and a metal layer on the insulatinglayer, said undercut cavity in said second wall electrically isolatingsaid second wall metal layer from the metal layers on said floor and onsaid first wall.